I think of the whole system operation like this... nothing really happens torque-vectoring wise until a wheel starts to slip. With the rear end disengaged, the torque being produced equally distributed between the 2 front wheels. Engaged it's equal to all 4 wheels. Under normal driving at part throttle in dry conditions, you're never outputting enough torque to break traction on any wheel whether it's 2 or all 4 engaged. The magic doesn't happen until something starts to slip.
A key thing is that the front axle cannot be disconnected from the transmission output ever. Well, except in the PTU neutral mode but that doesn't get used during driving. There is no clutch or anything to modulate how much torque is available to the front axle. This is why the system is said to be "FWD biased/based/etc". The front axle is engaged at all times through interlocked steel bits to the transmission output. The front axle has the POTENTIAL to put 100% of the available torque to the ground at any given time if traction allows, regardless of drive mode.
When Jeep mentions 40/60 or 60/40 power distribution, that's kind of generic-speak for the layperson.
Take sport mode... the reality is that 40 front/60 rear means that 60% of torque is AVAILABLE to the rear when in that mode. Front wheels on ice, ball bearings, a giant lube spill... the power transfer clutch is pressurized enough to allow 60% of the torque to flow to the rear axle. But this doesn't happen until there is front wheel/axle slip. All driven wheels are going to be equally taking the torque until one slips. With no wheel slip, there is also no slip at the power transfer clutch. Both sides of it are turning at equal speeds and there's no torque biasing going on. Of course the clutch slips a little when you're turning but that's a relatively small amount compared to what happens when a wheel spins on slick surface.
Or say you're on a dry road and punch it from a stop. In sport mode, so the rear is engaged and ready. For the first split second, there's not enough torque to break traction. Torque distribution is equal to all 4 wheels. Very soon though, as you get higher in the power band, a front wheel is logically going to try to break loose first. It'll start to slip but with 60% available to the rear through the transfer clutch, spin is going to be instantly arrested (like you might hear a little tire scrub up front but that's it) as the transfer clutch starts to slip. Realistically it probably only takes like 20% of available torque to be put down thru the rear axle to arrest any spinning up front because let's face it, we're not making 500HP here
Another way of saying it, the full 60% isn't going to go to the rear axle unless the front axle can only handle 40% (or less) of the torque being produced like on ice.
Snow mode is pretty much the same but flipped around so only 40% torque is available to the rear. They want you to have extra traction, but it's safer to not allow too much to be sent to the rear axle in order to prevent oversteer.
The 50/50 "split" in sand/mud or rock modes is pretty much just like a locked old-school transfer case. The power transfer clutch is fully pressurized, allowing 100% of available torque to go to either axle as ground traction dictates. That's why Jeep says "100% available to the rear axle". It's still 100% available to the front axle too though, like if you're dragging yourself up over a rock shelf and the rear is in the air.
I believe in rock mode it's allowed to slip a little, as the owners manual calls out "greater steering ability" in rock mode. That implies some allowed slip between the axles so there is less tire scrubbing on high traction rocks.
And of course on top of all this, the transfer clutch is fully variable so I'm sure the computer is able to shuffle the torque transfer ratio around in the various drive modes like auto/sport/snow depending on what it detects is going on traction and stability-wise.